What do VCs read to keep up to date with the market and find new trends? Which are the most popular websites?

If you are looking for data take a look at Mattermark which allows you to analyze the data on private companies and funding trends. You can get a free trial which would allow you to see all the data. Two other sites that contains a lot VC data are AngelList and Crunchbase.

If you are looking more for continual curation of analysis from industry leaders then I would recommend the Mattermark Daily. It's a daily newsletter curated by Nick Frost and the Mattermark team with the day's best posts from both investors and operators. It's free and is an absolute must read.

Like everyone said, mattermark. Especially if you're working on a project and not doing it professionally. You can also follow crunchbase or websites like built in chicago. They will give you insight into companies who have raised recently and how much.

However true VC's aren't looking at websites to follow trends, they are looking for patterns in the market that haven't yet emerged and are looking to follow a path. As example Google Ventures has pushed heavily into healthcare tech, now you're starting to see a lot of follow-on investors.